Conventionally, as a film forming apparatus to form a film on a surface of a semiconductor substrate, e.g., a silicon wafer (hereinafter, referred to as wafer for simplicity), there has been known the film forming apparatus which forms a film chemically (refer to Japanese Patent Laid-Open Application No. 2002-151489). However, when the film is formed on the wafer continuously by using the film forming apparatus above-described, there is a problem that a deposit which is adhered to an inner wall of the chamber, a susceptor installed in the chamber and the like is separated from the inner wall of the chamber and the susceptor, thereby polluting the wafer.
Consequently, it is needed to clean the film forming apparatus periodically. However, there are still the drawbacks that conditions for the cleaning are not established such that the cleaning thereof is not performed efficiently.